The present invention relates to an illuminating device incorporated in a copier for illuminating the entire image surface of a document laid on a glass platen by a plurality of flash lamps.
An illuminating device for the above-described application usually has a single or a plurality of flash lamps. Documents usable with a copier include one having a fraction of another document, or patch, adhered thereto. This kind of document is thicker in the portion thereof where a patch exists than the other portion. Therefore, when a document with a patch is illuminated by a conventional illuminating device using flash lamps, the edges of the patch, i.e., the steps between the patch and the base of the document appear on a reproduction in a line pattern or a stripe pattern. In light of this, it has been customary to locate each flash lamp at a particular position for thereby eliminating the shadows ascribable to the edges of the patch. This implementation, however, increases the size of the illuminating device and, therefore, the overall size of the copier. Although a predetermined voltage is applied to the individual lamps so that they may emit light at the same time, it is difficult to set the quantities of light accurately. Moreover, since the degree of deterioration and the service life differs from one lamp to another, the quantities of light to issue from the lamps are brought out of balance due to aging, preventing a document from being uniformly illuminated.